This project is designed to examine the effects of the formation of afferent and efferent connections on naturally occurring cell death in the principal sensory nucleus of the trigeminal nerve (PSN). The first step has been to determine the period during which PSN neurons were generated. This was accomplished by administering (3H) thymidine to fetuses on one day during the period from gestational day (GD) 10 to the day birth. Small neurons were generated on GD 12-14. Early-generated neurons tended to be disposed dorso-medially and late-generated neurons tended to be distributed in the ventro-lateral region. Thus, neuronal generation apparently proceeds by an inside-to-outside progression. On the other hand, large neurons were generated on GD 15. These neurons were distributed throughout the nucleus, but predominantly throughout the dorsal half of the PSN. From GD 17 to the day of birth, glia were generated, however, it is interesting to note that a few neurons also were born during this period. Our data also show that the numbers of neurons in the nucleus changes dramatically over the period from GD 16 to postnatal day (PD) 4. The number more than doubles from GD 16 to GD 20 and then begins to decrease about the day of birth. This decrease is due to naturally occurring cell death. The increase in the number results from the incorporation of post-migratory neurons into the PSN. Thus far, our autoradiographic data shows that neurons generated on GD 12 complete their migration by GD 18 and those generated on GD 14 complete their migration by the day of birth.